1. Field of the Invention
The present invention relates to communication on a wireless interface between stations, and in particular, to wireless communication between stations via multiple beams. The described solution may be used, e.g., in a cellular communication system serving mobile users or any other communication system wherein information can be communicated wirelessly on multiple beams between at least two stations.
2. Background of the Invention
A wireless communication system is a facility that enables communication via a wireless interface between two or more station entities such as base stations, user equipment and/or other transmitting and/or receiving nodes. The stations are typically provided with antenna means of some kind for enabling the transmission and/or reception of signals. The communication may include, for example, communication of voice, data, multimedia and so on.
An example of the wireless communication systems is a public land mobile network (PLMN). A PLMN is typically a cellular system wherein a base transceiver station (BTS) or similar access entity serves user equipment (UE) such as mobile stations (MS) via a wireless interface between these entities. Other examples of the wireless communication systems include the wireless local area network (W-LAN) and mobile communication systems that are at least partially based on use of communication satellites. Although the W-LAN based systems typically provide smaller coverage and the satellite-based system may provide larger coverage than the PLMN systems, the basic principle is the same—information is communicated between the stations on a wireless interface.
Communication between stations can be based on appropriate communication protocols and standards. For example, the communication system may be based on use of wideband code division multiple access (WCDMA), time division multiple access (TDMA), or any other appropriate access technique. The manner the wireless interface between the stations is to be arranged is defined by appropriate standards. Examples of mobile communication standards and/or specifications include, without being limited to these, specifications such as GSM (Global System for Mobile communications), GPRS (General Packet Radio Service), EDGE (Enhanced Data rate for GSM Evolution), AMPS (American Mobile Phone System), DAMPS (Digital AMPS), or 3rd generation (3G) communication systems such as the Universal Mobile Telecommunication System (UMTS), CDMA 2000, i-phone and so on.
Mobile communication services offered by mobile communication systems have become very popular among various types of users. Thus the quantity of user equipment has increased radically during the last few years. In addition to the conventional communication of voice (e.g. telephone calls), various data communication applications are also becoming more popular.
The increase in quantity of user equipment subscribing to a communication system and introduction of new type of services has led to capacity problems, especially during the so-called peak times. Furthermore, growing public demand for high data rate services, such as multimedia services, means that the capacity provided by conventional mobile communication systems is not always sufficient.
A proposal for increasing the capacity in wireless communication systems is to transmit information signals between base stations and user equipment in parallel via several transmit antenna elements and thus via several wireless communication channels or paths.
The multi-channel communication can be provided by means of multiple beams. The multiple beams can be provided by means of an antenna array comprising multiple antenna elements. The multiple antenna elements may be provided with adapted transmission and detection techniques. The multiple beams may be provided as so called fixed beams or by means of so called “smart antenna” arrangements. The smart antenna enables beam forming such that it is possible to form and direct the beams appropriately.
Introduction of multiple beams is believed to provide a significant increase in the spectral efficiency of wireless interfaces compared to conventional single beam antenna links.
A more specific proposal for the multi-antenna arrangements is the so called Multiple Input/Multiple Output (MIMO). The MIMO proposals promise linear increase in the link capacity. The prevailing view has been that the capacity increase is due to richness in the propagation environment, and that ideally the multiple transmit and receive antennae should be uncorrelated. Thus a MIMO system would typically have omni-directional transmit and receive antennae that are apart from each other and transmit with equal power in the whole cell or sector. Each MIMO channel includes a stationary correlation structure.
The proposed MIMO systems require computation of the so-called matrix channel between the transmitting and receiving stations. The MIMO systems may employ feedback arrangements wherein the feedback is used in the construction of optimal beams. The construction is done by decomposing each matrix channel to a set of eigenbeams.
In MIMO systems channel estimates have to be acquired for all channels. That is, when a stationary correlation structure is present in each channel, the structure of the modes to be transmitted is solved by eigenanalysis of each of the channels. To construct these matrices (i.e. the eigenbeams), relatively complex algorithms have to be used.
A further problem that may appear in the fixed multiple beam arrangements relates to selection of multiple fixed beams for the transmission. More particularly, a problem situation may be caused since a base station conventionally just measures the received power from a pilot signal transmitted by a user equipment and then chooses those fixed beams that receive the highest powers. In the fixed beam arrangements it is possible that neighbouring fixed beams are selected for a user, as these are likely to have fairly similar signal propagation conditions and thus almost equal powers. Because of this it is possible that the neighbouring fixed multiple beams are selected. The neighboring beams may interfere with each other.
A further problem that also relates to the above discussed transmissions on neighbouring beams relates to difficulty in determining whether the signals propagate as shown in FIG. 5 or as shown in FIG. 6. As shown in FIG. 5, signals may propagate from the base station 10 to the user equipment 1 on the overlapping portions of beams 40 and 41. FIG. 6 shows another scenario wherein two signals transmitted on the adjacent beams 40 and 41 are received by the user equipment 1, the user equipment being located such that the line of sight conditions are blocked by an obstacle 8. A problem in here is that it is not possible for the base station 10 to know, based solely on power level measurements of user equipment pilot signals, whether a signal from the user equipment that has components from two adjacent fixed beams is a result of two distinct signal paths (FIG. 6) or just one signal path (FIG. 5) that arrives from direction that is between two adjacent fixed beams.